Pneumatic gun SPR-01 "Alligator"

[popgun pete]

Interesting to see that in Russia a close copy of the Cressi-Sub "Star" has been manufactured for a number of years; the "Alligator" or SPR-01. The shapes of the moulded plastic parts are nearly identical to the Cressi-Sub product, although the line release pivot pin is located in a small protuberance on the bottom of the trigger finger guard rather than pressed through the frame of the finger guard itself. Most significant difference is the inner barrel is 14.8 mm in diameter! This would be an interesting gun to convert to pneumo-vacuum operation, however the forum comments that I have read about it have cast doubts on the quality of the gun. This may be quality of materials or assembly, or both, however it is difficult to find an objective evaluation of the gun as not all comments may be of an informed nature. The specifications for the gun are below:

Length: 500 mm, 600 mm, 700mm
Weight: 0,75 kg, 0,8 kg, 0,7 kg
Force when charging: 205-225 N, 225-255 N, 225-255 N
Max. chamber pressure gun: 2.35 MPa
Flight range of the harpoon: not less than 5 meters​

Options: spear tip - 2 pieces, spare harpoon - 2 pieces, trident - 1pc, pump, nylon cord, a user manual.

The photos are from a web-site that sells the "Alligator" speargun.

 

[popgun pete]

After delving into the http://apox.ru forums I found that besides the usual corrosion problems with steel spear shafts and "occasional" air leaks that the gun's trigger has the undesirable tendency to not always hold! The "Alligator" speargun has the usual tipping "see-saw" sear lever operated by a short transmission pin that penetrates the pressure vessel. This sear lever actually functions in the same way that the old style single-piece trigger in band guns operates, only in the pneumatic gun the "trigger" has been flattened out as rather than your finger pulling on a vertical trigger hook that job is carried out by the transmission pin pushing up from below onto the extended rear end of the sear lever. The plastic external trigger provides a leverage advantage to this release system as your finger pulls some distance from the trigger's pivot pin whereas the transmission pin is pushed from a position very close to the trigger's pivot pin. It is a simple system that has worked for decades, so how have the Russians stuffed it up? A photo of the sear lever and piston shows the answer (see attached), the tip of the sear tooth catch is too small and it grips a mushroom head that has a correspondingly small ledge. If it does not fully click into place then the sear lever will have only a tenuous grasp on the piston tail. To fix it the piston tail and sear lever need greater contact areas to provide a better engagement.
One aspect often not mentioned is the need to stop the sliding piston wobbling in the inner barrel as deflection of the piston can make it lift off the sear tooth and escape before you pull the trigger, like when you have just stopped pushing on the shaft! Piston wobbles can also momentarily break seal contact and allow pressurized air to escape from the gun in small bursts. That is why these pistons have tended to be long and have at least two widely spaced seals, but in modern times the plastic piston body has meant that the forward rubber seal can be deleted and replaced by a close fitting section that contacts the inner barrel wall instead. In the early Mares "Sten" guns, using all metal pistons, the rubber piston seals were back to back (actually one combined seal) and the pistons could wobble, so Mares used to put a moulded plastic piece inside the gun that acted like a sear box housing located inside the inner barrel tube at the rear. The top of the piston tail's mushroom head was held closely by this plastic item (it effectively put a roof over the top of it) so that it stopped the piston tail tilting off the sear tooth and possibly escaping. Once Mares went to using three seal pistons, two rubber cone type seals (often described as wiper seals, but they did more than that) and a centre "O" ring spaced out along the piston, they stopped fitting this plastic "internal sear box housing", so few people today would have seen one. Installing an item like this would solve some of the "Alligator" gun's latch retention problems, however it would be preferable to increase the size of the catch surfaces as the plastic housing can eventually wear out and increase the clearance which it is intended to minimize.